All patents, patent applications, and publications cited within this application are incorporated herein by reference to the same extent as if each individual patent, patent application or publication was specifically and individually incorporated by reference.
The invention relates generally to the use of photolithographic and dry etching techniques to provide microstructures in polymers. The invention also relates to methods for fabricating optical polymer waveguide devices.
The use of photolithographic and dry etching techniques to define microstructures in inorganic and organic materials is well known to those skilled in the arts of microelectronics, semi-conductors, and photonics. Typically, referring to FIG. 1, a microstructure is produced in a material by a process of 1) exposing a photoresist composition (10), which can be any one of commercially available photoresists such as those containing novolak resins, on a material (12) disposed on a substrate (14) to radiation (16) through a photomask (18) that shields part of the photoresist from the radiation, whereby the parts of the composition that are exposed to radiation (20) change chemically relative to the parts that are shielded from the radiation (22); 2) removing one of the parts of the photoresist layer (also referred to as “developing”), thereby uncovering portions of the material (12), and forming the pattern of the photomask on the material (12); 3) treating the uncovered portions of material (12) and remaining photoresist portions (22) with a reactive ion plasma (24) to remove portions of material (12), thereby transferring the pattern of the photomask to the substrate (14); 5) removing the remaining photoresist (22) (also referred to as “stripping”) to leave behind the material (12), thereby transferring the pattern of the photomask to substrate (14).
Making microstructures is a crucial step in fabricating optical polymer waveguides that can be used in optical communications systems and methods. An optical polymer waveguide is comprised of at least one core region (also referred to as the waveguiding core or core) that is surrounded by a cladding region (also referred to as cladding or clads). Most of the light that travels through the waveguide resides in the core while a smaller portion resides in the cladding. Photolithography and dry etching have been used to make optical polymer waveguides, for example see U.S. Pat. Nos. 5,263,111; 5,381,506; 6,061,487; and 6,306,563. More specifically, photolithography and dry etching are used to make, referring to FIG. 2, rib core (28), quasi-rib core (30), or trench core (32) either on or in a clad (34). Referring to FIG. 3, a typical process of making a waveguide core comprises: 1) providing at least a four layer structure including a photoresist (36), core polymer (38), clad polymer (40), and substrate (42); 2) exposing the photoresist to radiation (44) through a photomask (46); 3) developing the waveguide pattern in the photoresist (36) on the core polymer (38); 4) dry etching the core polymer (38) and photoresist (36) with a reactive ion plasma (39) to provide a waveguide core (38) covered with the remaining photoresist (36); and 5) removing the remaining photoresist to provide the waveguide core (38) on the clad polymer (40). Since the photoresist is dry etched during the etching of the core polymer, the photoresist layer typically is thicker than the final waveguide core thickness.
In a typical waveguide core making process, problems may be encountered in the photolithography step when the photoresist has a thickness greater than about 1 μm. Longer exposure times, normally used in thicker photoresist, can amplify light scattering, diffraction and back reflection phenomena. One solution of this problem can be the use of anti-reflective coatings; however, anti-reflective coatings require curing temperatures in excess of 180° C., which will cause loss of electro-optic activity in poled polymers. Thus, during a process of making an electro-optically active polymer waveguide core, it is desirable to use a thin photoresist layer in order to minimize problems from long exposure times while avoiding the use of anti-reflective coatings that would cause loss of electro-optic activity.
Using a thin layer of photoresist to make thick structures can be accomplished using a hardmask that functions as a dry etch stop, which is known in the field of microelectronics, for example see U.S. Pat. Nos. 5,219,788; 5,370,969; and 6,019,906. A hardmask is referred to sometimes as a “hardmask layer,” “barrier layer,” “etch stop,” or “interfacial film.” The hardmask acts as an etch stop during the polymer dry etching step. A process for making microstructures using a hardmask is illustrated in FIG. 4. First, a four layer structure is provided that comprises a thin layer of photoresist (48), a polymer (50) to be patterned on a substrate (52), and a hardmask layer (54) between the polymer and photoresist. The photoresist is exposed to radiation (56) through a photomask (58) and developed to give the pattern of remaining photoresist (48) on the hardmask layer (54). The hardmask layer (54) is then dry etched with a reactive ion plasma (59) to remove portions of the hardmask layer (54). The polymer and photoresist are then selectively dry etched with a second reactive ion plasma (60) that does not etch the hardmask layer (54) to form a pattern consisting of the remaining hardmask layer (54) and the remaining polymer layer (50) on the substrate (52). If desired, the hardmask layer may be removed with either dry or wet etching. Since the hardmask acts as a dry etch stop during etching of the polymer, the photoresist layer does not have to be thicker than the polymer layer, and can be around or less than 1 μm thick.
Current microelectronics hardmasking techniques cannot be applied to fabricating electro-optically active waveguides since the hardmask is deposited by chemical vapor deposition (CVD). CVD requires temperatures typically in excess of 200° C., which can cause loss of electro-optic activity in most poled second-order nonlinear optical (NLO) polymers due to either chromophore decomposition or thermal randomization of the aligned chromophores. The development and uses of second-order NLO chromophores, including polymer matrix development, waveguide fabrication, and optical device fabrication are described in U.S. Pat. Nos. 5,272,218; 5,276,745; 5,286,872; 5,288,816; 5,290,485; 5,290,630; 5,290,824; 5,291,574; 5,298,588; 5,310,918; 5,312,565; 5,322,986; 5,326,661; 5,334,333; 5,338,481; 5,352,566; 5,354,511; 5,359,072; 5,360,582; 5,371,173; 5,371,817; 5,374,734; 5,381,507; 5,383,050; 5,384,378; 5,384,883; 5,387,629; 5,395,556; 5,397,508; 5,397,642; 5,399,664; 5,403,936; 5,405,926; 5,406,406; 5,408,009; 5,410,630; 5,414,791; 5,418,871; 5,420,172; 5,443,895; 5,434,699; 5,442,089; 5,443,758; 5,445,854; 5,447,662; 5,460,907; 5,465,310; 5,466,397; 5,467,421; 5,483,005; 5,484,550; 5,484,821; 5,500,156; 5,501,821; 5,507,974; 5,514,799; 5,514,807; 5,517,350; 5,520,968; 5,521,277; 5,526,450; 5,532,320; 5,534,201; 5,534,613; 5,535,048; 5,536,866; 5,547,705; 5,547,763; 5,557,699; 5,561,733; 5,578,251; 5,588,083; 5,594,075; 5,604,038; 5,604,292; 5,605,726; 5,612,387; 5,622,654; 5,633,337; 5,637,717; 5,649,045; 5,663,308; 5,670,090; 5,670,091; 5,670,603; 5,676,884; 5,679,763; 5,688,906; 5,693,744; 5,707,544; 5,714,304; 5,718,845; 5,726,317; 5,729,641; 5,736,592; 5,738,806; 5,741,442; 5,745,613; 5,746,949; 5,759,447; 5,764,820; 5,770,121; 5,76,374; 5,776,375; 5,777,089; 5,783,306; 5,783,649; 5,800,733; 5,804,101; 5,807,974; 5,811,507; 5,830,988; 5,831,259; 5,834,100; 5,834,575; 5,837,783; 5,844,052; 5,847,032; 5,851,424; 5,851,427; 5,856,384; 5,861,976; 5,862,276; 5,872,882; 5,881,083; 5,882,785; 5,883,259; 5,889,131; 5,892,857; 5,901,259; 5,903,330; 5,908,916; 5,930,017; 5,930,412; 5,935,491; 5,937,115; 5,937,341; 5,940,417; 5,943,154; 5,943,464; 5,948,322; 5,948,915; 5,949,943; 5,953,469; 5,959,159; 5,959,756; 5,962,658; 5,963,683; 5,966,233; 5,970,185; 5,970,186; 5,982,958; 5,982,961; 5,985,084; 5,987,202; 5,993,700; 6,001,958; 6,005,058; 6,005,707; 6,013,748; 6,017,470; 6,020,457; 6,022,671; 6,025,453; 6,026,205; 6,031,945; 6,033,773; 6,033,774; 6,037,105; 6,041,157; 6,045,888; 6,047,095; 6,048,928; 6,051,722; 6,061,481; 6,061,487; 6,067,186; 6,072,920; 6,081,632; 6,081,634; 6,081,794; 6,086,794; 6,090,322; 6,091,879; and 6,210,867.
Another reason present microelectronics hardmasking techniques cannot be applied to fabricating either active or passive waveguides is that the hardmask requires removal by either wet etching or dry etching. Wet or dry etching of a hardmask deposited directly on a waveguide core material can cause surface roughness on the waveguide, which can lead to unacceptably high optical loss.